Methods in the drying of a paper web as well as dryer sections of a paper machine

Abstract: The invention concerns a method and a device in the drying of a paper web (W), wherein, after the press section (20) of the paper machine, the paper web (W) is dried in a number of successive groups (R 1 ...R N ;R S ) with single-wire draw, in which groups the contact-drying cylinders (10) are placed in the upper row and the reversing suction cylinders (11) or the equivalent suction rolls in the lower row or in equivalent diagonal or vertical rows. From the side of its lower face, across the entire length of the dryer section, the paper web (W) is dried by means of contact-drying cylinders (10). The paper web (W) is passed as a closed draw from one group (R 1 ...R N ; R S ) with single-wire draw to the next group, and the paper web (W) is guided, while it is placed on the drying wire (15) at the side of the outside curve, by means of the reversing suction cylinders (11) with a curve radius D 2 /2, which is chosen in the range of D 2 /2 ≈ 250...1000 mm. The paper web (W) is kept in constant contact with the drying wire (15), as it is placed at the side of the outside curve, against the effect of centrifugal forces by means of a difference in pressure, which difference in pressure is preferably extended over the entire inner circumference of said reversing suction cylinders (11). In addition to the above, or as an alternative, the paper web (W) is dried from the side of its upper face on a draw (W 1 ) or draws of the paper web (W) that is/are free from the wire (15) and/or a flow of drying air is applied to the upper face of the paper web (W) through the drying wire (15) and/or on said draws (W 1 ) of the paper web (W) that are free from the wire (15).

Description: The invention concerns a method in the drying of a paper web, in which method, after the press section of the paper machine, the paper web is dried in a number of successive groups with single-wire draw, in which groups the contact-drying cylinders are placed in the upper row and the reversing suction cylinders or equivalent suction rolls in the lower row or in equivalent diagonal or vertical rows, and in which method the paper web is pressed by means of the drying wire against the heated faces of the contact-drying cylinders, and the paper web is passed, in each group with single-wire draw, on support of the same drying wire from one contact-drying cylinder onto the next contact-drying cylinder over the reversing suction cylinders or equivalent suction rolls, and, when the paper web is placed at the side of the outside curve on the drying wire, the web is held on the wire by means of a difference in pressure against the effect of centrifugal forces. Further, the invention concerns a dryer section in a paper machine, which dryer section is composed of a number of successive so-called normal groups with single-wire draw, in which groups the contact-drying cylinders are placed in the upper row and/or the reversing suction cylinders are placed in the lower row and/or in equivalent diagonal and/or vertical rows, and between which normal groups the paper web to be dried has closed group-gap draws, and which reversing suction cylinders have been arranged so that their outer circumferences are subjected to negative pressure. In the way known from the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw, the groups of drying cylinders include two wires, which press the web, one from above and the other one from below, against the heated cylinder faces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible of fluttering, which may cause web breaks, in particular since the web is still relatively moist and, therefore, of low strength. This is why, in recent years, increasing use has been made of said single-wire draw, in which each group of drying cylinders has one drying wire only, on whose support the web runs through the whole group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, whereas, on the reversing cylinders or rolls between the drying cylinders, the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are placed outside the wire loop and the reversing cylinders or rolls inside said loop. In prior art normal groups with single-wire draw, the heated drying cylinders are placed in the upper row and the reversing cylinders in the lower row, which rows are, as a rule, horizontal and parallel to one another. In the applicant's FI Patent No. 54,627 (equivalent to US Patent No. 4,202,113) it is suggested that the above normal groups with single-wire draw and so-called inverted groups with single-wire draw be placed one after the other, in which said inverted groups the heated drying cylinders are placed in the lower row and the reversing suction cylinders or rolls in the upper row, the principal objective being to dry the web symmetrically from both of its sides. There is used a grooved cylinder inside a wire loop of a single-wire draw in order to increase drying efficiency. The set-direction moisture profile is controlled by the inverted groups. Further, the drying of the web takes place entirely while the web is against the wire. Also, Messrs. Beloit Corp. have made some suggestions for a dryer section that comprises normal and inverted cylinder groups, in which respect reference is made to the published international patent applications WO 88/06204 and WO 88/06205. In the following, when the terms normal (drying) group" and inverted (drying) group" are used, what is meant is expressly the cylinder groups with single-wire draw of the sort mentioned above for multi-cylinder dryers. In dryer sections that comprise inverted and normal drying groups, various problems have occurred, for which problems the present invention is supposed to suggest novel efficient solutions. These problems have been encountered in the runnability of the dryer section and in the threading of the web, problems arising from differences in the speeds of different wires, problems in the removal of broke especially in inverted groups, as well as problems related to the symmetry of the drying of the web in the z-direction arid problems related to the control of transverse shrinkage of the web. As a rule, these problems tend to become worse as the running speed of the paper machine becomes higher. With respect to the prior art involved in and related to the present invention, reference is made to the following patent publications and articles published in journals: W. Haessner, Trocknungstechnik und deren Entwicklung"; Das Papier 44, 1OA, 1990 The Valmet Sym-Run Concept", Paper Asia, May/Jun 1992 J. Yli-Kauppila, Dryer Section for High Speed Paper Machines", Proceedings of the Helsinki Symposium of Alternate Methods of Pulp and Paper Drying, Helsinki June 4-7, 1991 S. Palazzolo, No-draw drying", Tappi Journal, September 1990 W. Leitenberger, Die Contirun-Trockenpartie fur schnellen, sicheren Bahnlauf", Das Papier, Heft 6, 1992 US Patents Nos. 3,753,298; 3,868,780; 4,602,439; 4,972,608; 4,982,513; 5,022,163; 5,065,529; 5,146,696, and US-5,177,880 V. Korhonen and A. Kuhasalo, Ropeless tail threading from press to reel", World Pulp & Paper Technology 1993 H. Lepistö und P. Eskelinen, Verbesserung der Lauffähigkeit schneller Papiermaschinen mit Hilfe neuer Ventilationseinrichtungen", Das Papier 1985, Heft 10A Lindberg, Juppi, Eskelinen, High Speed Dryer Section Developments for Sheet Stability", 79th Annual Meeting, Technical Section CPPA, 1992. With respect to the prior art closely related to the invention, reference is made further to the applicant's FT Patent Application No. 906216, in which a method is described in the dryer section of a paper machine, in particular for reduction of a tendency of curling of paper, in which method the paper web is dried on drying cylinders, against whose heated faces the paper web is pressed by means of a drying wire, and in which dryer section groups of drying cylinders are used, in which twin-wire draw and/or single-wire draw is/are applied. In this method, it has been considered novel that, in the dryer section, hot water steam is fed substantially onto the entire width of the paper web, by means of which steam tensions that have been formed or that tend to be formed in the fibre mesh in the paper web are relaxed by means of heat and moisture in the area of their formation or substantially immediately thereafter. Further, in said FT Pat. Appl. 906216, a dryer section of a paper machine is described, which is intended for carrying out the above method, which comprises one, or preferably several successive drying groups which consist of drying cylinders and of wire guide rolls and/or reversing cylinders, and in which dryer section single-wire draw and/or twin-wire draw is/are applied. In this dryer section, it has been considered novel that one or several steam supply boxes is/are fitted in the dryer section, which box/boxes extend(s) substantially across the entire transverse width of the paper web to be steam-treated, and which steam box/boxes communicate(s) with a steam source, and that the steam box/boxes comprise(s) a counter-face, which, together with the free face of the paper web that runs at its proximity, forms a contact-free steam-treatment gap. From the US 5,033,207 it is known to blow hot air against the lower face of the web while the web is supported by the wire. The purpose of said blowings is to dry the web further. However, this proposed blowing does not equalize the z-direction moisture gradient, but makes it worse. The WO 93/22497 teaches using, for control of the z-direction moisture profile and curling, a short twin-wire draw group placed after a long single-wire draw portion and having separate regulation of steam pressure in connection with upper and lower cylinders. The control of curling (twin-wire draw) is a prior known technique and all drying applied to the web is carried out when the web lies against the wire. It is known from the US 4,625,430 to use a coolable cylinder in a single-wire draw in order to increase drying efficiency. Earlier, the applicant noticed that the drying capacity of a dryer section is increased when the supplies of steam into the lower cylinders in the initial end of a dryer section consisting of groups with twin-wire draw are closed. This phenomenon comes from removal of the water present between the fibres out of the web into the drying wire by means of condensation, the condensation being the more intensive, the colder the wire. When the process moves towards the dry end of the dryer section, where the water between the fibres has already been removed (dry solids content Ka > 70... 75%), the nature of the process of removal of water out of the web is changed. In the dry end, the water is boiled out of the interior of the fibres, and the rest of the water is boiled off from the fibre walls, after which the water evaporates into the air from the free face of the web in the area of the reversing suction cylinders or is condensed on the drying cylinders into the wire. However, it is essential that the temperature of the web rises to a level higher than 100° C, for the water bound in the fibres does not yet even boil exactly at 100° C. On the other hand, it is known that the temperature of the web is lowered in the area of the reversing suction cylinders below 100° C as a result of evaporation. When the web arrives on a drying cylinder, heating is carried out first, after which vaporization of the water out of the fibres starts, the water separated from the fibres being evaporated into the air in the gap between the drying cylinder and the reversing suction cylinder and on the reversing suction cylinder. It is probable that, in view of the evaporation, the gap between a reversing suction cylinder and a drying cylinder is relatively ineffective in the final end of the dryer section, compared with the gap between a drying cylinder and a reversing suction cylinder. As a rule, a tendency of curling of the web in the dryer section arises from asymmetry of the distribution of moisture in the z-direction of the web. In such a case, in a normal prior-art Sym-Run"™, on the upper face of the web more water remains in the fibres than on the lower face, which lower face is heated on the cylinder and is allowed to be evaporated as a free face in the areas of the reversing suction cylinders. If the distribution of moisture in the z-direction in the web is asymmetric after the drying, later, the side of the web that has been dried to a greater extent absorbs more water out of the air into itself, in which connection curling of the web occurs. The coefficients of thermal expansion of paper are, in the transverse direction, (8...6) *10 -4 % / °C, while the hydroexpansion coefficient of water absorption is (77...237) * 10 -4 % / % RH. These figures, for their part, illustrate the significance of the distribution of moisture in the z-direction in a web. Further, it is known in the prior art, in a dryer section, to use devices for regulation of the transverse moisture profile, such as infrared and/or moistening devices, but in the prior art, by means of these devices, attempts have not been made to control the moisture profile of the paper in the z-direction, i.e. in the direction of thickness, but they are used exclusively for the control of the transverse moisture profile of the web. Further, in SC paper machines, a procedure is known from the prior art in which the paper web is dried to en excessive dryness in order to obtain a sufficiently good moisture profile, whereupon the paper web is re-moistened to a moisture content optimal in view of the calendering process. Thus, the function of these moistening devices is just to increase the ultimate moisture content of the paper, and not to equalize its moisture profile in the z-direction. According to present-day knowledge, the direction and extent of curling of a paper web is determined primarily by the direction of evaporation of water taking place in the dryer section. In the prior art, a dryer section is known which is exclusively composed of the above groups with single-wire draw, but in these groups, between the contact-drying cylinders placed in the upper rows in the groups, normal small diameter suction rolls that are provided with inside suction boxes have been used. The dryer section concerned has been supplied by Messrs. J.M. Voith GmbH, and its site is PM 1, Stora Feldmuehle, Reisholz, Duesseldorf, Germany. A drawback of these rolls is the high requirement of negative pressure and suction energy, because, owing to the small roll diameter, on these rolls, high centrifugal forces arise that tend to separate the web from the drying wire. By means of the curve sectors of small radius, said suction rolls also produce a rather large relative difference in speed between the drying wire and the web, which is in many respects unfavourable. Further drawbacks are the wear of the seals at the suction box inside the suction rolls and the repeated requirement of servicing of said seals as well as the high noise level. This prior-art dryer section has also required development in the respect that, since on the contact-drying cylinders, in all the groups with single-wire draw, the drying effect is applied to one side of the web only, i.e. to the lower side of the web, the web tends to be dried asymmetrically in the z-direction, and to a greater extent at the web side placed in contact with the faces of the contact-drying cylinders. Thus, one object of the present invention is to suggest novel solutions for these problems. In the following, the problems and requirements of further development that have occurred in the prior-art dryer sections, for example, according to the patents and papers mentioned above, will still be dealt with concisely. As a background, it should be stated that the highest web speeds of paper machines are currently already of an order of 25 m/s (metres per second), but before long even the speed range of 25...40 m/s will be taken to use. In such a case, a bottle-neck for the runnability of a paper machine will, to an increasing extent, consist of the dryer section, which will, moreover, with the use of the prior-art dryer concepts, become quite long. In the inverted drying groups mentioned above, in the case of breaks, a problem consists of the removal of broke, for inverted groups are not self-cleaning by the force of gravity. This why the times taken by breaks in an inverted group are substantially longer than in normal groups. Thus, the object of the present invention is to provide a dryer section in which no inverted groups are needed at all, but which, yet, meets the other requirements that are imposed. The above problems and some other problems are emphasized further if, in the groups with single-wire draw, prior-art small-diameter suction rolls proper are used that are provided with an inside suction box. In order to eliminate this problem, in some machines, it has been even necessary to open some group gaps and to lower the level of negative pressure in the suction rolls. It is a further object of the present invention to provide methods and dryer sections whose runnability can be brought to a particularly high level. Further, it is an object of the invention to provide methods and dryer sections in which so-called ropeless tail threading can be applied favourably across the entire length of the dryer section in the machine direction, which contributes to simpler constructions and to shorter standstill times. It is a non-indispensable further object of the invention to provide a dryer section whose length in the machine direction can be made at least to some extent shorter, whereby the cost of investment of the paper machine and of the paper machine hall can be lowered. It is a particular object of the invention to provide a method and a dryer section after which the paper that has been dried has a sufficiently symmetric moisture profile in the z-direction, in view of the purpose of use and the other properties of the paper. In relation to the above, an object of the invention is to provide novel methods and devices for the control of curling of a paper web. It is a further object of the invention to provide a method and a dryer section in which it is possible to control the transverse curling and/or moisture profile of the paper web and by whose means any tensions that have arisen or tend to arise in the fibre mesh in the paper web can be relaxed by means of heat and/or moisture. In view of achieving the objectives stated above and those that will come out later, the method of the invention is mainly characterized in that the paper web is dried from the side of its lower face substantially across the entire length of the dryer section by means of said contact-drying cylinders, that the paper web is dried from the side of its upper face on a draw or draws of the paper web that is/are free from the wire, thereby the drying of the paper web from the side of its upper face is promoted. In a particularly advantageous application of the first embodiment of the method of the invention, the reversing suction cylinders, in particular the applicant's "VAC-ROLL"™ cylinders, are arranged to be heated preferably starting from the dry solids content of the web Ka > 70...75%. In this mode of application, the following particular advantages are obtained: The tendency of curling of the web is reduced, because the web is heated and dried also from the top side. The web enters into the gap after the reversing suction cylinder and onto the following drying cylinder hotter than in the prior art, in which case the evaporation is more efficient right from the beginning of the contact sector on the drying cylinder. The heating of the reversing suction cylinders can be accomplished by means of a number of alternative techniques. In such a case, it is possible to employ an induction heater fitted above the reversing suction cylinder, by means of which heater the outer face of the mantle of the reversing suction cylinder is heated to, for example, about 150° C, thermal energy being transferred from the outer face of the cylinder mantle through the drying wire into the web. Moreover, or alternatively, the induction heater can be arranged as a static construction inside the mantles of the reversing suction cylinders. Moreover, or alternatively, in connection with the inner face of the reversing suction cylinders, a steam coil can be arranged, through which coil steam is blown, the heat of the steam being conducted into the cylinder mantle and from there further through the wire into the web. Also, it is possible to use various resistive heating arrangements, which are based on the use of electrical resistors, for example, so that electrical resistors are fitted inside the reversing suction cylinder and/or the projecting portions between the grooves in the outer face of the reversing suction cylinder are provided with electrical resistors, or said projecting portions, which pass around the cylinder mantle in ring form or in spiral form, are fitted as heating resistors as a whole, the heating current being passed into said resistors, for example, via slide rings and brushes. In some forms of application of the invention, it is possible to employ regulation of the quantity, temperature and/or moisture level of the blow air for UNO-RUN-BLOW-BOXes™ and/or transverse profiling of the web, by means of which operations, besides the moisture profiling, a contribution is also given to the control of the symmetry of drying of the web in the z-direction. Also, by means of the permeability of the drying wire, it is possible to control the symmetry of drying of the web in the z-direction by regulating the proportion of evaporation taking place on the face of the drying cylinder in the overall evaporation. Also, the tension of the drying wire is a further regulation parameter that can be used in the invention, for example, so that, in the last group or groups, in particular adjustable tensions of the drying wire are used, and thereby the mutual ratios of the evaporations taking place through the top face and the lower face of the web are affected exactly in the area that is critical in view of the curling of the web, as a rule, in the range of dry solids content Ka > 70...75%. A draw or draws of the paper web free from the wire can be arranged favourably in the area of the gaps between the wire groups and/or inside the wire groups, for example, so that, on one or several drying cylinders, the drying wire is guided by means of a particular guide roll out of contact with the web and with the face of the drying cylinder and is returned onto the same drying cylinder. The dryer section in accordance with the invention is mainly characterized in that the dryer section is primarily composed of said normal groups with single-wire draw, in which groups heated reversing cylinders and/or drying-radiation devices and/or means for blowing of drying gas have been fitted, by whose means a substantial drying impulse can be applied to the upper face of the paper web so as to equalize the drying profile of the paper web in the z-direction and, if necessary, also to increase the drying capacity of the dryer section. The moisture profile of the web in the z-direction, i.e. in the thickness direction, is equalized by means of heated reversing suction cylinders, by means of particular other drying devices, and/or by means of a moistening device. These drying devices can be accomplished, e.g., as gas or electric infrared radiators, and in the moistening devices it is possible to use moist air and/or water steam blown against the free face of the web to be dried. As was stated above, for said purpose, the reversing suction cylinders can be arranged to be heated by means of various arrangements of equipment, preferably to a temperature of about 150°C. In such a case, it is possible to employ induction heaters fitted on the free outer sector and/or inside the mantle of the reversing suction cylinder, electric devices based on resistive heating and/or steam supply devices, of which more detailed exemplifying embodiments will be described later. It is also possible to use microwave dryers and radio-frequency RF-dryers as said devices that equalize the moisture in the direction of thickness. The drying and/or moistening arrangements mentioned above and acting upon the web through the upper face are preferably placed in the area of the final end of the dryer section, where the dry solids content Ka of the web is Ka > 65 %, as a rule Ka > 70...75%. Said arrangements can be favourably connected with arrangements for the control of the transverse moisture profile of the web. In the present invention, by means of a combination of a number of process steps and solutions of construction that are partly known in themselves in the prior art, it has been possible to create a dryer section that is more advantageous both in respect of its construction and in respect of its runnability, the paper produced by means of said dryer section having quality properties that meet even high requirements, also in respect of symmetry and dimensional stability. In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated schematically in the figures in the accompanying drawing, the invention being by no means strictly confined to the details of said embodiments. Figure 1 is a schematic side view of a dryer section in accordance with the invention that makes use of the method of the invention. In Fig. 1, the press section and the initial part of the dryer section are shown above, and the final end of the dryer section underneath, and the plane of section through the dryer section is denoted with A-A. Figure 2 illustrates a group-gap draw between two normal groups, wherein an infrared dryer is employed on the first contact-drying sector in the latter group. Figure 3 shows a preferred embodiment of the last group or groups in a dryer section in accordance with the invention. Figure 4 is an illustration corresponding to Fig. 3 of a second preferred embodiment of the last group or groups in a dryer section in accordance with the invention. Figure 5 is a schematic vertical sectional view in the machine direction of an exemplifying embodiment of a heated reversing cylinder applied in the invention, wherein electrical-resistor heating fitted inside the cylinder mantle is employed. Figure 6 is an illustration corresponding to Fig. 5 of a heated reversing cylinder in which the heating is accomplished by means of an arrangement of steam-supply coil fitted inside the cylinder mantle. Figure 7 is a sectional view in the cross direction of a mantle of a reversing cylinder in whose grooved face the projecting portions between the grooves are provided with electrical heating resistors. Figure 8 shows a group with twin-wire draw, which is the last group in the dryer section and which can be possibly applied especially in a method in accordance with the first embodiment of the invention in particular exceptional cases. Figure 9 shows such a dryer section in accordance with the invention in which the so-called normal groups have been accomplished as diagonal groups. As is shown in Fig. 1, the paper web W in is passed into the dryer section from the press section onto the drying wire 15 of the first group R 1 with single-wire draw, to which wire the web is made to adhere by the effect of the negative pressure in the suction boxes 13. Fig. 1 schematically shows the press section 20, which precedes the dryer section, in which press the web W is dewatered by pressing it by means of three successive roll nips NP 1 , NP 2 and NP 3 before the web W is transferred into the first group R 1 with single-wire draw in the dryer section. The dryer section comprises 6 groups R 1 ...R 6 with single-wire draw, in whose group gaps the web W has a closed draw. The dryer section in accordance with the invention comprises normal groups R l ...R N , usually N = 4...l0, preferably N = 5...7, and typically N = 6. All the groups R 1 ...R N with single-wire draw are so-called normal groups, in which, e.g., steam-heated smooth-faced drying cylinders 10 are placed in the upper horizontal row and the reversing suction cylinders 11 are placed in the lower horizontal row. In the last normal group R 6 the last two upper cylinders 10' and one reversing suction cylinder 11', which is placed between said two upper cylinders 10', are placed by the dimension H 1 higher than in the preceding groups R 1 ...R 5 . The dimension H 1 is typically H 1 ≈ 400 mm. The frame part 100 of the dryer section is illustrated quite schematically. Each normal group R 1 ...R N has a drying wire 15 of its own, which is guided by guide rolls 18. The drying wires 15 press the web W to be dried on the drying cylinders 10 against their smooth heated faces, and on the reversing cylinders 11 the web W remains on the outer face of the wire 15 at the side of the outside curve. On the reversing cylinders 11 the web W is held reliably on support of the wire 15 against the effect of centrifugal forces by the effect of the negative pressure present in the grooved faces 12 of the reversing suction cylinders 11, whereby transverse shrinkage of the web W is also prevented. As the reversing suction cylinders 11, particularly favourably, the suction cylinders are used that are marketed by the applicant under the trade mark VAC-ROLL"™, which cylinders have no inside suction box and in respect of the details of whose constructions reference is made to the applicant's FI-Patent No. 83,680 (equivalent to US Pat. No. 5,022,163). In a preferred embodiment of the invention, the support contact between the web W and the drying web 15 is kept adequate also on the straight runs between the drying cylinders 10 and the reversing cylinders 11, at least on the runs taking place from the drying cylinders 10 to the reversing cylinders 11, by making use of blow-suction boxes 17, by whose means formation of pressures induced by the wire 15 is also prevented in the closing wedge-shaped nip spaces between the wire 15 and the cylinder 11 mantles. With respect to the details of the constructions of these blow-suction boxes 17, which are marketed by the applicant under the trade mark UNO RUN BLOW BOX"™, reference is made to the applicant's FI Patents Nos. 59,637, 65,460, and 80,491 (equivalent to US Pats. 4,441,263, 4,516,330 and 4,905,380). After the introduction of the UNO RUN BLOW BOX"™ in the market, the applicant's competitors have also suggested some blow-box solutions, with respect to which reference is made to the US Patents 4,502,231 (J.M. Voith GmbH) and 4,661,198 (Beloit Corp.), and the applications of said blow boxes in the positions of the blow boxes 17 are also included in the scope of the overall concept of the present invention. In the groups R 1 ...R N with single-wire draw, also in the gaps between the reversing cylinders 11, blow boxes 16 are used, by whose means said gap spaces are air-conditioned and evaporation from the web W is promoted. The faces of the drying cylinders 10 are kept clean by doctors 14,24. In the invention, it is a further essential feature and advantage that, in the groups R 1 ...R N with single-wire draw, which extend across the entire length of the dryer section, removal of broke by gravity can be applied, for the groups R 1 ...R N with single-wire draw are open downwards, so that the broke paper web WS can be removed without special arrangements onto the broke conveyor (not shown) placed in the basement space of the paper machine, and on the broke conveyor further into the pulper or pulpers. In Fig. 1, the overall horizontal length of the dryer section in the machine direction is ∼ 70 m when six normal groups R 1 ...R N (N = 6) are used. The number N 1 of the drying cylinders 10 used in each of the individual normal groups R 1 ...R N is in the range of N 1 = 3...8, preferably N 1 = 4...7. In view of prevention of transverse shrinkage of the web W, it is of particular importance that the web W should be held in reliable contact with the drying wires 15 all the time. This holding effect is achieved on the reversing cylinders 11 by means of the negative pressure present in the grooved mantles 12 of the outer faces of said cylinders, and on the straight runs between the cylinders 10 and the reversing cylinders 11 by means of the pressure levels arranged by means of the blow-suction boxes 17. Fig. 2 shows an infrared radiation dryer 30 arranged in accordance with the second embodiment of the invention, which dryer is placed between the last group R N with single-wire draw and the next to the last group R N-1 with single-wire draw. The dryer 30 applies infra-red radiation IR, in the area of the drying gap 31, to the upper face of the web W 1 that is placed free on the face of the drying cylinder 10a, i.e. to the side of the web W that is opposite to the web face that is placed in contact with the drying cylinders 10, 10a. Said area W 1 of the web W that is free from the wire has been provided by guiding the drying wires 15a and 15b by means of guide rolls 18a and 18b so that a free area W 1 of the web W is formed but that, nevertheless, a closed draw is accomplished from the group R N-1 to the next group R N . The infrared radiator 30 extends across the entire width of the web W. The infrared radiator 30 may operate either with electric energy or with gas energy. In Fig. 2, the regulation means are shown schematically as a block 32, by means of which regulation means both the power level P T of the infrared radiation IR and its distribution P P in the transverse direction are regulated. By means of the distribution P P of the power, the transverse moisture profile of the web W is controlled. It is an essential feature of the operation of said infrared radiator 30 that it equalizes the moisture profile of the web W in the z-direction by to the upper face of the web W applying a substantial impulse of drying energy. IR-devices 30 can be placed in one or several group gaps R n -R n+1 . In Fig. 1, it is illustrated schematically that IR-devices 30 have been placed in the group gaps between the last three groups R 4 , R 5 and R 6 and additionally inside the groups R 3 and R 4 . Moreover, by means of said IR-devices 30, it is possible to increase the drying capacity of the dryer section so that the overall length of the dryer section can be shortened by a few drying cylinders. In stead of the IR-devices 30 described above, it is also possible to use corresponding microwave or RF-radiators. In stead of, or in addition to, the radiators 30, it is possible to use devices for blowing of drying air, by whose means, in the free areas W 1 of the paper web, drying-air jets are applied to the upper face of the web W so as to intensify the evaporation. In Fig. 2, the reference numeral 30A in parentheses refers to these blow devices, by whose means air jets F are applied to the upper face of the web W in its free area W 1 . In Fig. 2, a hydraulic cylinder 30a is shown schematically, which is fitted in connection with the dryer 30;30A and by whose means the dryer 30;30A can be shifted further apart from the paper web W, e.g., for the time of threading of the web W and/or in order to facilitate the removal of broke, which may be necessitated by a web break. In the following, reference being made to Figs. 3 to 7, such exemplifying embodiments of methods and equipment, especially in accordance with the first embodiment of the invention, will be described in which reversing suction cylinders 11 with heated mantle faces are used in order to improve the symmetry of drying of the paper web W in the z-direction and to reduce the tendency of curling of the paper web W, and, if necessary, also to increase the drying capacity so that the overall length of the dryer section can be reduced by up to ∼ 10 per cent. According to Fig. 3, the last group R N in the dryer section is provided with devices 45 fitted on the free sectors at the top side of the reversing suction cylinders 11, said devices comprising induction heaters 46, which have been integrated with the UNO-RUN-BLOW-BOX"™ blow devices 17 described above. The induction heaters 46 have an appropriately small air gap 46 in relation to the outer faces of the mantles 50 of the cylinders 11 or VAC" rolls. The induction heating effect is illustrated by the references IND. By means of the induction heaters 46, in the mantle faces of the cylinders II or VAC" rolls, a resistive heating effect is produced, which is based on an eddy current effect and by whose means the temperature of the cylinder mantle 50 is raised preferably to about 150° C. Then, the mantle faces of the cylinders 11 or VAC" rolls heat the web W through the drying wire 15 and increase the symmetry of drying in the z-direction of the web W and also intensify the drying on the drying cylinders 10 and on the straight runs between said drying cylinders and the reversing suction cylinders 11. Induction heaters 46 and equivalent are used preferably in the final end of the dryer section in an area in which the dry solids content Ka of the web W is Ka > 70...75%, which area is critical in view of the formation of the tendency of curling of the web W. In such a case, groups provided with heated reversing suction cylinders 11 are used at least as the last group R N in the dryer section or as the last two groups. In view of regulation of the heating effect in the cross direction of the web W, the heating effect of the induction heaters 46 can be arranged adjustable in the cross direction, which is illustrated schematically by the block 37P, by whose means the heating currents I 1...S passed into the various blocks 1...S in the cross direction of the device 45 are regulated. In a corresponding way, the blow devices 17 can be divided into blocks 1...M in the cross direction of the web W, into each of which blocks air blows F 1...M adjustable in respect of their humidity, quantity and/or temperature can be passed, said air flows being controlled by means of the block 17P illustrated schematically. The regulation arrangements 37P, 17P can be provided in connection with one or several devices 45. Said transverse profiles of the heating and/or blow effects do not always necessarily require regulation, in which case the blocks 37P and 17P illustrate the regulation of the levels of heating or blow effects without particular regulation of the profile, in which case the profile can be even or be pre-set as of a different form. According to Fig. 4, the last group R N or the last two groups R N and R N-1 in the dryer section is/are provided with heated reversing suction cylinders 11, in which, in the interior space V of their mantles 50, which space is subjected to a vacuum, induction heaters 48 are fitted, which have an air gap 49 sufficiently small in view of the inductive heating effect IND in relation to the inner face of the cylinder mantle 50. If necessary, one or several inductive heaters 48 can be divided into blocks 1...S in the cross direction of the web W, the heating currents I 1...S that are fed into said blocks being regulated by means of the unit 38P for the purpose of regulation of the level and/or the transverse profile of the heating effect. As regards the level and/or the profiling of the heating effect, the induction heaters 46,48 shown in Figs. 3 and 4 can also be accomplished in the form of induction heaters known per se and used in paper machine technology, reference being made, in respect of the details of their constructions, to the applicant's FI Patent No. 76,260 (equivalent to the US Patent No. 4,675,487). According to Fig. 4, in the final end of the group R N , inside the loop of the drying wire 15, a second particular fabric 150 is fitted, which is guided by the guide rolls 180. The permeability of the fabric 150 is very low, or the fabric 150 is completely impermeable. By means of the fabric 150, evaporation is prevented on the cylinders 10K, and in this way the symmetry of the drying of the web W in the z-direction is promoted further. In stead of, or in addition to, the use of the additional fabric 150, it is also possible to use an impermeable drying wire 15 or a drying wire that has a very low permeability, by means of which wire evaporation is reduced in the area of the drying cylinders 10 and symmetry of the web W drying W in the z-direction is promoted. Fig. 5 shows a heated reversing cylinder 11 whose mantle 50 is, in a way in itself known, provided with outside grooves 12 and with perforations 51 passing through the mantle 50, said perforations opening from outside into the grooves 12 and from inside into the interior space V of the cylinder 11, which space V is subjected to a vacuum. Inside the mantle 50, axial electrical resistors 55 are fitted, which resistors are attached to a suitable insulating support arrangement (not shown) and which resistors are connected in series and/or in parallel, and to which resistors the heating current is passed through slide rings and brushes (not shown) placed at the ends of the cylinders, said heating current heating the mantle 50 from inside. Similarly to Fig. 5, Fig. 6 shows such an arrangement of heating of a reversing suction cylinder 11 as comprises a number of axial steam supply pipes 56. These pipes 56 can heat the cylinder mantle 50 based on conduction of heat. Fig. 7 is an axial sectional view in the cross direction of the machine of the cylinder mantle 50 of a heated reversing suction cylinder 11. In a way in itself known, the mantle 50 is provided with through perforations 51. The widened outer orifices 51a of the holes 51 are opened into the grooves 12 in the outer face of the cylinder 11 mantle. The grooves are defined by annular pieces 52, which are attached into the grooves 53 in the outside face of the mantle 50. The annular pieces 52 can be substituted for by a corresponding spiral-shaped continuous part. The annular pieces 52 are made of a suitable metal or insulating material. Inside the pieces 52, electrical resistors 54 are fitted, which are connected in adjacent pieces 52 in parallel and/or in series. If the annular pieces 52 are made of a metal, the electrical resistors 54 must be insulated from the pieces 52. The heating current is passed to the resistors 54 through slide rings or brushes (not shown) placed in connection with the axle journal or journals of the cylinder 11. The outer faces of the pieces 52 constitute the outer face 11' of the cylinder mantle 11 between the grooves 12, against which face 11' the drying wire 15 is in contact, the web W being placed on its outer face. The raised temperature of the pieces 52 is conducted through the drying wire 15 to the web W and applies drying to the web W from the side of its upper face, thus promoting the symmetry of the web in the z-direction. Many other arrangements of drying on reversing suction rolls 11, different from Figs. 3 to 7, are also possible. In addition to the locations in the group gaps, Fig. 1 shows dryers 30 as placed inside the wire groups R 3 and R 4 , said dryers being placed in such free areas W 1 of the web W as have been provided by guiding the drying wire 15 apart from the drying cylinder 10 and from the web W by means of a particular guide roll 18' so that the runs 15' of the drying wire 15 thus formed define a "pocket", in which the dryer 30 is placed to apply a drying effect to the upper face in the free area W 1 of the web W. Fig. 1 shows an air-blow unit 35 as fitted inside the loop of the wire 15 of the next to the last group R 5 , which unit 35 has a blow gap 37 in relation to the adjacent drying cylinder 10b. Through the air intake pipe 36 of the blow unit 35, dry and hot air is introduced into the unit 35, which air is blown in the treatment gap 37 against the wire 15 so that the air that is blown ventilates the pores m the wire 15 and lowers the component pressure of steam present in them, thereby intensifying the evaporation taking place on the face of the cylinder 10b from the upper face of the paper web W. In this way, the moisture profile of the web W in the z-direction can be equalized and, moreover, the overall drying capacity of the paper machine can be increased. Said blow units 35 may be fitted in connection with more groups R N than one, and one or several groups may also include more than one blow units 35. In Fig. 1 it is shown that moistening devices 40 have been fitted underneath the groups R 5 and R 6 , which devices have a treatment gap 42 with the adjacent web W and with the reversing suction cylinder 11B. Said moistening device 40 may be, for example, a steam box in itself known or a device that blows moist air and/or water mist, by whose means the moisture profile of the web in the z-direction is equalized by blowing a moist medium onto the lower face of the web that has been dried in contact drying on the drying cylinders 10. By means of the moistening devices 40, in a way in itself known, it is also possible to equalize the transverse moisture profile of the web W and, if necessary, also to relax the internal tensions in the web in accordance with the principles that are described, e.g., in the applicant's FI Patent Application No. 906216 (equivalent to US Pat. Appl. No. 07/808,161), and thereby to control the curling profile of the paper. There may be even several such moistening devices 40 in different groups R N , and such devices are preferably placed in the last group R N or in the last two groups R N or R N-1 . Said moistening devices 40 are preferably placed in the final end of the dryer section in an area in which the dry solids content Ka is Ka > 65%, preferably in an area in which the dry solids content is Ka > 80%. In view of the runnability, the blow devices 17 mentioned above are also quite important, said blow devices being placed on the runs of the drying wire 15 and of the web W passing from the drying cylinders 10 to the reversing suction cylinders. These boxes 17 are used preferably in the initial end of the dryer section only, when the dry solids content Ka is Ka < about 70%. The primary function of the dryers 30,35,45,49 described above is to equalize the moisture profile of the web in the z-direction by application of drying energy expressly from the side of the upper face of the web W, i.e. from the side opposite to the side that is in contact with the hot faces of the drying cylinders 10. However, it is a further advantage of said dryers 30,35,45,49 that by their means it is possible to increase the drying capacity even to such an extent that the length of the dryer section can be reduced even by about 5...10 m in comparison to a solution in which contact-drying cylinders 10 alone are used. In the present invention, it is favourably possible to apply so-called ropeless tail threading. Ropeless threading can be accomplished in the normal groups R 1 ...R N on the drying wires 15 and on the reversing suction cylinders 11 as well as on the straight runs of the wires 15 in their connection by means of blow boxes 17 and by subjecting the reversing suction cylinders 11 to negative pressure. Further, in connection with the doctors 14,24, it is possible to install air-blow devices, by whose means separation of the leader strip from the cylinder face 10 and its adhering to the drying wire 15 can be ensured. As to the dimensioning of the various cylinders and rolls in the dryer section, it should be stated that advantageously the diameters D 1 of the drying cylinders 10 in the normal groups R 1 ...R N ;R S are chosen as D 1 ≤ 2.5 m, preferably D 1 ≈ l.8...2.2 m. The diameter D 2 of the reversing suction cylinders 11 is chosen as D 2 ≈ 0.5...2 m, preferably in the range of D 2 ≈ 1.0...1.5 m, particularly appropriately in the range of D 2 ≈ 1.2...1.5 m. The diameter D2 range of D 2 ≈ 0.5...l.2 is, as a rule, employed in narrower paper machines only. Fig. 2 also shows the horizontal distance A 0 between the cylinders in a normal group R 1 ...R N , which is A 0 ≈ 2100 mm, and the vertical distance H 0 between the cylinders 10,11 is H 0 = 1600 mm. The diameter D 3 of the guide rolls 18,18a,18b is typically in the range of D 3 ≈ 400...700 mm, depending on the width of the machine. When the diameter D 2 of the reversing suction cylinders 11 is chosen in the way mentioned above, the centrifugal forces that attempt to separate the paper web W from the drying wire 15 on the turning sectors of the reversing suction rolls 11 can be made so low that, with reasonable levels of negative pressure in the grooved face 12, the paper web W can be kept reliably in contact with the drying wire 15 across the entire length and width of the dryer section. In this way, transverse and longitudinal shrinkage of the paper web W is prevented, whereby the properties of quality of the paper are improved substantially. The reversing suction cylinders 11 are preferably accomplished without inside suction boxes. With the prerequisites given above, the negative pressure in the groove spaces 12 in the cylinder mantle of the reversing suction rolls 11 is, as a rule, arranged to be preferably in the range of 1...3 kPa. By means of said level of negative pressure, both reliable holding of the web W on the drying wire 15 on the turning sectors of the reversing suction cylinders 11 against the effects of centrifugal forces and, in general, a reliable contact of support between the web W and the drying wire can be ensured, so that transverse shrinkage of the web W and the resulting problems of curling and fibre orientation can be avoided. Even though above mainly such embodiments of the invention have been described in which all the reversing suction cylinders 11 are large-diameter (D 2 ≈ 800...2000 mm) suction cylinders with no inside suction box, in which the suction zone extends across the entire outer circumference of the mantle, it should be emphasized that the scope of the invention also includes embodiments in which some of said reversing suction cylinders 11 have been substituted for by so-called normal small-diameter suction rolls that are provided with inside suction boxes, the diameter of said rolls being, as a rule, smaller than the above diameter D 2 (typically 500...1200 mm). If said normal suction rolls, whose suction zone usually extends over the sector covered by the paper and the wire, are used, they are preferably placed in the initial end of the dryer section only. One of the regulation parameters that can be utilized in the invention and by whose means the progress of the drying can be controlled is the tensions T N of the drying wires 15;I5A,I5B;150. In a preferred embodiment of the invention, T N is chosen in the range of T N ≈ 1.5...8 kN/m. It is favourably possible to use such an arrangement of tension of the drying wires 15 in which, in the groups R 1...N ;R S , the tension T N of the wires 15;I5S is increased constantly as the drying makes progress, in accordance with the principles that are described in the applicant's FI Patent No. 83,441. Fig. 8 shows a last dryer group R DW in a dryer section, which group can be used in the method of the invention, in particular in a method in accordance with the first embodiment of the invention, in exceptional cases, and which dryer group R DW is, differing from the general idea of the invention, a twin-wire group. In the twin-wire group R DW , there are two rows of drying cylinders 10A and 10B placed one above the other. In connection with the upper cylinders 10A, there is an upper wire 15A guided by the guide rolls 18,18A. In a corresponding way, in the gaps between the lower drying cylinders 10B, there are guide rolls 18B and a lower wire I5B guided by said guide rolls 18B and by the guide rolls 18. The web W has free draws W F between the rows of cylinders 10A and 10B. Together with the drying wires 15A,15B and the free sectors of the cylinders 10A,10B, the free draws W F define pockets P, whose ventilation must be arranged sufficiently efficient. By means of the twin-wire group R DW , if necessary, the symmetry of drying in the z-direction can be improved further by, on the lower cylinders 10B, drying the web W to a greater extent from the side of its upper face than it is dried from the side of its lower face on the cylinders 10A. For this purpose, the steam pressure in the lower cylinders 10B can be arranged higher, the sector of covering by the web W can be made larger on the lower cylinders 10B, and or the tension T B of the lower wire 15B can be made larger than the tension T A of the upper wire 15A, in which case the tensioning force of the tensioning roll 18 is F B > F A . The free sectors of the cylinders 1OA and/or 10B can be provided, for example, with induction heaters similar to the devices 46 illustrated in Fig. 3. However, it should be emphasized that, as a rule, in the invention, it is unnecessary to use a group R DW with twin-wire draw, but the symmetry of drying in the z-direction can be achieved by the other means described above, in which case the free draws W F susceptible of fluttering are avoided and the dryer section can be arranged so, across its entire length, that it is cleansed from paper broke downwards by the force of gravity in the event of web breaks. On the other hand, in the area of the last group R DW , the web W is already quite dry and, therefore, also strong, so that, in particular with thicker grades, the free draws W F do not increase the risk of breaks decisively. On the other hand, free draws W F may be even useful in some cases, because in their area any inner strains in the web W can be relaxed. Thus, the most advantageous embodiments of the invention are, as a rule, carried into effect by using exclusively groups with single-wire draw. When the web W departs from the dryer section at W out , its dry solids content k out is, as a rule, in the range of k out ≈ 92.98%, whereas the dry solids content of the web W on its entrance into the dryer section is k in ≈ 40%...55%. Fig. 9 shows such a modification of the dryer section as shown in Fig. 1 in which all or some of the single-wire groups R 1...N have been substituted for by special groups RS 1 ,RS 2 ,RS 3 ... etc. of diagonal alignment, in which the first three contact-drying cylinders 10S are placed, in the direction of progress of the web W, in a downwards inclined plane T 1 , and the next three corresponding cylinders 10S in an upwards inclined plane T 2 . In Fig. 9, the reversing cylinders in the groups RS 1 ...RS 3 are denoted with the reference 11S and the reversing rolls with the reference 18S and the wires with the reference 15S. Said inclined groups may be preceded by normal single-wire groups R 1 ...R N similar to those shown in Fig. 1, the web W having closed draws between said normal groups and said inclined groups RS N-1 and RS N . In stead of the inclined groups RS, it is also possible to use vertical or almost vertical cylinder groups. With respect to said vertical groups, reference is made to the applicant's FI Patents Nos. 53,333 and 82,097 (equivalent to US Pats. Nos. 3,868,780 and 4,972,608) and to the US Patent No.5,177,880 of Messrs. J.M. Voith GmbH. The diagonal groups RS or the corresponding vertical groups, at least their lower portions, may extend to below the floor level of the paper machine hall into its basement spaces. When diagonal groups RS or equivalent vertical groups are used, when necessary, for example, with paper grades thicker than average, it is also possible to use a twin-wire group R DW as shown in Fig. 8 as the last single group, which is, however, as a rule, neither advantageous nor necessary. The scope of the invention also includes embodiments in which the overall length of the dryer section has been shortened in respect of the groups R 1 ...R N with single-wire draw by, in one or several groups R 1 ...R N , arranging the drying cylinders 10 in two or more horizontal, vertical, or inclined planes. In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from what has been stated above for the sake of example only.

Claims: Method in the drying of a paper web (W), in which method, after the press section (20) of the paper machine, the paper web (W) is dried in a number of successive groups (R 1 ...R N ;R S ) with single-wire draw, in which groups the contact-drying cylinders (10) are placed in the upper row and the reversing suction cylinders (11) or equivalent suction rolls in the lower row or in equivalent diagonal or vertical rows, and in which method the paper web (W) is pressed by means of the drying wire (15) against the heated faces of the contact-drying cylinders (10), and the paper web (W) is passed, in each group (R 1 ...R N ) with single-wire draw, on support of the same drying wire (15) from one contact-drying cylinder (10) onto the next contact-drying cylinder over the reversing suction cylinders (11) or equivalent suction rolls, and, when the paper web (W) is placed at the side of the outside curve on the drying wire (15), the web (W) is held on the wire by means of a difference in pressure against the effect of centrifugal forces, characterized in that the paper web (W) is dried from the side of its lower face substantially across the entire length of the dryer section by means of said contact-drying cylinders (10), that the paper web (W) is dried from the side of its upper face on a draw (W 1 ) or draws of the paper web (W) that is/are free from the wire (15), and thereby the drying of the paper web (W) from the side of its upper face is promoted. Method as claimed in claim 1, characterized in that the cylinder mantles (50) of the reversing suction cylinders (11) are heated in the last group (R N ) or groups with single-wired draw in the dryer section in the area in which the dry solids content Ka of the web (W) is Ka > 70...75%, and/or that, in the area of the last group (R N ) with single-wire draw, the temperature of the outer face of the cylinder mantles (50) of the reversing suction cylinders (11) is raised to the range of 80...250° C, preferably to the range of 120...170° C. Method as claimed in claim 1 or 2, characterized in that the heating of the cylinder mantles of the reversing suction cylinders (11) is carried out by means of induction heaters (45;48), which are fitted outside (Fig. 3) and/or inside (Fig. 4) the cylinder mantle (50). Method as claimed in claim 1 or 2, characterized in that the heating of the cylinder mantles (50) of the reversing suction cylinders (11) is carried out by means of resistive electrical-resistor heating by placing the heating resistors in connection with the inner face (Fig. 5) and/or the outer face (Fig. 7) of the cylinder mantle (50). Method as claimed in claim 1 or 2, characterized in that the heating of the cylinder mantles (50) of the reversing suction cylinders (11) is carried out by means of hot water steam (Fig. 6). Method as claimed in any of the claims 1 to 5, characterized in that the paper web (W) is dried from the side of its upper face on a free draw (W 1 ) or draws of the paper web (W) that is/are free from the wire (15) and that has/have been arranged in the area of the gaps between the wire groups. Method as claimed in any of the claims 1 or 6, characterized in that the paper web (W) is dried from the side of its upper face on a free draw (W 1 ) or draws of the paper web (W) that is/are free from the wire (15') and that has/have been arranged inside the wire groups by guiding the drying wire (15') apart from, and back onto, the drying cylinder (10) by means of a particular wire guide roll (18'). Method as claimed in any of the claims l to 7, characterized in that in the method, in addition to the contact-cylinder drying, drying energy is employed by whose means, besides equalization of the moisture profile of the paper web (W) in the z-direction, the drying capacity of the dryer section is increased and a shortening of the dryer section is permitted, in comparison with a solution in which contact-cylinder drying alone is employed. Method as claimed in any of the claims 1 to 8, characterized in that on the draws (W 1 ) of the paper web (W) that are free from the drying wire (15), a field of infra-red radiation (IR) is applied to the upper face of the web, which field is produced by means of electric energy or gas energy. Method as claimed in any of the claims 1 to 9, characterized in that onto said draws (W 1 ) of the paper web (W) that are free from the drying wire (15), dry and hot air (Fa,Fb) is blown in order to promote the evaporation taking place from the upper face of the paper web (W) and to equalize the drying profile of the web in the z-direction. Method as claimed in any of the claims 1 to 10, characterized in that in the method, onto the lower face of the paper web (W), preferably when the web runs over said reversing suction cylinders (11B), a moist medium is fed, preferably moist air and/or water mist, so as to equalize the moisture profile of the paper web (W) in the z-direction. Method as claimed in any of the claims 1 to 11, characterized in that in the method, a moist medium, preferably moist air and/or water mist, is fed onto the lower face of the paper web (W) so as to control, preferably to equalize, the transverse moisture profile of the paper web (W). Method as claimed in any of the claims 1 to 12, characterized in that in the method, the paper web (W) is kept, across the entire length and width of the dryer section, reliably in contact with the drying wire (15) so that transverse and longitudinal shrinkage of the paper web (W) are substantially excluded, whereby the properties of quality of the paper are improved. Method as claimed in any of the claims 1 to 13, characterized in that in the method, the symmetry of drying in the z-direction is increased by, in the last group (R N ) or groups with single-wire draw, using a drying wire (15) that is impermeable to air or whose permeability is very low, by means of which wire evaporation is prevented on the turning sectors of the drying cylinders (10). Method as claimed in any of the claims 1 to 14, characterized in that in the last group (R N ) or groups with single-wire draw in the dryer section, a second fabric loop (150) is used, which is fitted inside the drying-wire loop (15) and which is impermeable to air or whose permeability is very low, and by means of which fabric loop (150) evaporation is reduced or prevented on the drying cylinders (10K) placed in connection with said fabric loop (Fig. 4). Method as claimed in any of the claims 1 to l5, characterized in that the symmetry of drying in the z-direction of the web (W) is increased by regulating the humidity level of the air blown into the closing inlet nips of the drying wire and the reversing suction cylinders in the last group (R N ) with single-wire draw. Method as claimed in any of the claims 1 to l6, characterized in that in the method, as the last group (R N ), a group (R DW ) with twin-wire draw is used, which comprises two rows of drying cylinders (10A,10B) placed one above the other and in which group (R DW ) the web (W) is dried on the lower cylinders (10B) to a greater extent than on the upper cylinders (10A) by, in the lower cylinders (10B), employing a higher steam pressure and/or a higher tension (T B ) of the lower wire (15B), or any other, equivalent arrangements (Fig. 8). Method as claimed in any of the claims 1 to 17, characterized in that across the entire length of the paper machine, broke removal by the effect of the force of gravity is employed through the spaces in the normal wire groups (R 1 ...R N ;R S ) that are open downwards. Method as claimed in any of the claims 1 to 18, characterized in that substantially across the entire length of the dryer section, ropeless tail threading of the paper web is employed, which is aided by air-blow devices (13,16,17). Dryer section in a paper machine, which dryer section is composed of a number of successive so-called normal groups (R 1 ...R N ) with single-wire draw, in which groups the contact-drying cylinders (10) are placed in the upper row and/or the reversing suction cylinders (11) are placed in the lower row and/or in equivalent diagonal and/or vertical rows, and between which normal groups (R 1 ...R N ;R S ) the paper web to be dried has closed group-gap draws, and which reversing suction cylinders (11) have been arranged so that their outer circumferences are subjected to negative pressure, characterized in that the dryer section is primarily composed of said normal groups (R 1 ...R N; R S ) with single-wire draw, in which groups heated reversing cylinders (11) and/or drying-radiation devices (30) and/or means (30A) for blowing of drying gas (F) have been fitted, by whose means a substantial drying impulse can be applied to the upper face of the paper web (W) so as to equalize the drying profile of the paper web (W) in the z-direction and, if necessary, also to increase the drying capacity of the dryer section. Dryer section as claimed in claim 20, characterized in that in the last group (R N ) with single-wire draw or in the last two groups (R N and R N-1 ) with single-wire draw in the dryer section, there are reversing suction cylinders (11) which are provided with heating devices, the heating of the cylinder mantles (50) of said cylinders being accomplished by means of induction heaters (46), which are fitted outside the cylinder mantle and which are preferably integrated with air blow devices (17). Dryer section as claimed in claim 20, characterized in that in view of heating the cylinder mantles (50) of the reversing suction cylinders (11), induction heaters (48) (Fig. 4), resistive electrical heaters (55) (Fig. 5), or means (56) for the supply of hot water steam (Fig. 6) have been fitted inside the cylinder mantles (50). Dryer section as claimed in claim 20, characterized in that in view of heating the reversing suction cylinders (11), the cylinder mantles (50) of the reversing suction cylinders are provided with an outside resistive heating-resistor arrangement, preferably of a sort in which the projection parts (52) which define the grooves (12) in the outer face of the cylinder mantle (50) contain an electrical heating-resistor arrangement (54) (Fig. 7). Dryer section as claimed in any of the claims 20 to 23, characterized in that on one or several drying cylinders (10;10A), for the paper web (W), an upper face (W 1 ) of the web (W) that is free from the drying wire (15) has been arranged in the area(s) of one or several wire-group gaps. Dryer section as claimed in any of the claims 20 to 24, characterized in that on one or several drying cylinders (10;10A), for the paper web (W), an upper face (W 1 ) of the web (W) that is free from the drying wire (15) has been arranged inside one or several dryer groups (R 3 ,R 4 ) by guiding the drying wire (15') apart from the drying cylinder (10) and back again onto the cylinder by means of a particular wire guide roll (18'). Dryer section as claimed in any of the claims 20 to 25, characterized in that in one or several groups (R 1 ...R N ;R S ) with single-wire draw, in connection with a contact-drying cylinder (10;20S) or cylinders, a blower device (35) is fitted, which has a treatment gap (37) in relation to the adjacent drying wire (15) and to the paper web (W) placed below said wire, and from which blower device (35) a flow of drying air can be applied into the pores in said drying wire (15), whereby evaporation of water from the paper web (W) can be promoted. Dryer section as claimed in any of the claims 20 to 26, characterized in that the dryer section comprises means (40,41,42) for the supply of a moist medium, preferably moist air and/or water mist, from which means the moist medium can be applied so as to equalize the moisture profile of the paper web (W) in the z-direction. Dryer section as claimed in any of the claims 20 to 27, characterized in that the dryer section comprises means (40,41,42) for the supply of a moist medium, preferably moist air and/or water mist, from which means the moist medium can be applied so as to control, preferably to equalize, the transverse moisture profile of the paper web (W). Dryer section as claimed in any of the claims 20 to 28, characterized in that said dryer devices (30,30A) and/or said means (40,41, 42) for the supply of a moist medium have been arranged so that they can be shifted by means of an actuator (30a) further apart from the paper web (W) for the time of tail threading and/or so as to facilitate the removal of broke necessitated by a web break.

Inventors: KAIHOVIRTA JUHA, KERTTULA REIMA, YLI-KAUPPILA JOUKO, KAIHOVIRTA, JUHA, KERTTULA, REIMA, YLI-KAUPPILA, JOUKO
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1994
Main classification: D21F5
IPC codes: D21F5, D21G7, F26B13